1. Field of the Invention
The present invention relates to a receiver coil array for a magnetic resonance imaging (MRI) system, particularly to a receiver coil array of an MRI system with a decoupling circuit.
2. Description of the Prior Art
Magnetic resonance imaging (MRI) is an imaging modality, wherein the signals arising from the resonance of atomic nuclei within a magnetic field are reconstructed for imaging. More specifically, the basic principle of magnetic resonance is that for an atomic nucleus with an in odd number of protons, e.g. the hydrogen nucleus that is widespread in a human body, the positively charged protons thereof exhibit spinning movements that create a magnetic moment, serving as a small magnet. The arrangement of the self-spin axes of the small magnet normally is irregular. In a homogenous strong magnetic field, however, the self-spin axes of the small magnet will be rearranged depending on the direction of the magnetic lines. In this condition, excited by the radio frequency (RF) pulse at a specific frequency, the hydrogen nucleus, serving as a small magnet, absorbs a certain amount of RF energy so as to resonate, so the magnetic resonance phenomenon occurs. Once the transmission of RF pulses is stopped, the excited hydrogen nucleus will release the absorbed energy gradually, and then its phase and energy level will resume to the state before the excitation. Different from the imaging principle of X ray, CT, etc., MRI causes no radiation harm to human body, and therefore it has provided a broad research field for clinical applications.
Magnetic resonance imaging system basically include a basic field magnet that generates the aforementioned homogenous magnetic field, a gradient magnetic field coil system, an RF coil system, a control unit that operates the gradient coil system and the RF coil system to execute a scanning sequence, and an image processing and displaying system. The gradient coil system is employed to modify the main magnetic field, and to generate a gradient magnetic field. The gradient magnetic field allows three-dimensional coding of the magnetic resonance signals in human body as to spatial orientation, though the magnetic field strength thereof is only one of several hundredths of the main magnetic field. The RF coil includes a transmitter coil and a receiver coil. The transmitter coil transmits pulses into human bodies with a proper RF pulse for excitation, serving as a short wave transmitting station and a transmitting antenna, whereas the hydrogen nucleus (atomic nucleus with an odd number of protons) within the human body receive the pulse, serving as a radio receiver. After the transmission of the pulses (excitation) is stopped, the hydrogen nucleus within the human body serves as a shortwave transmitting station, whereas the MR signal receiver serves as a radio receiver to receive magnetic resonance signals. The functions of the magnetic resonance signal receiver are realized by the receiver coils.
In order to improve the configuration of the system, the receiver coil in an MRI system is usually formed of multiple channels, which herein refers to the number of the terminals capable of receiving signals in the system, and there may be one or more receiver coils in a channel. At the same time, in the MRI system, in order to increase the signal-to-noise ratio (SNR) and enlarge the field of view (FOV) at the diagnosis (imaged) site, the most effective method is to introduce a number of loop coils into the design of the receiver coil, also known as a coil array. When using such loop coils, the coupling between the coils is relatively large. Coupling is a type of interference in the circuit. The coupling not only results in noise and affects the received signal, but also may reduce the life span of the receiver coil due to excessive induced current. Therefore, it is necessary to decouple these loop coils. Generally, however, decoupling can be performed only between two loop coils, and it is very difficult to perform decoupling among three or more loop coils.
In order to solve the problem of decoupling three loop coils, one of the conventional methods is to connect the two outer loop coils inversely. FIG. 1 shows a schematic diagram of such connection, wherein the solid line is a saddle coil, and the coordinate axis shows the size of the receiver coil in meters (m); the dashed line is a receiver coil which is folded into the shape of an “8”, so that the coil is divided into two parts represented by Ring1 and Ring2 as shown respectively with L as the central coil. The receiver coils Ring 1, L, and Ring2 can be regarded as three receiver coils, with the two loop coils Ring 1 and Ring2 sharing the same voltage since they are actually formed of the same wire (conductor), and their magnetic fluxes are offset at the center. Therefore there is no coupling between the two coils, and coupling only exists between Ring1 and L, L and Ring2. Since coupling between two coils can be easily counteracted by decoupling with existing technology, decoupling of three coils is simplified into the decoupling between two coils.
The above-described solution still has certain drawbacks. First, since the magnetic flux is proportional to the signal-to-noise ratio (SNR), and the magnetic fluxes at the center of the two outer loop coils (Ring1, Ring2) are offset with respect to each other, the signal-to-noise ratio of the signals received at the center of the coil is not improved. Furthermore, since the outer Figure 8-shaped coils are actually the same coil, the voltage applied to the Figure 8-shaped coils equals that applied to the central coil L, but the length of the outer coils is about twice that of the central coil L. The longer the coil the larger the resistance the coil has, so the strength of the received signal of the outer coils is weak, therefore the improvement of the signal-to-noise ratio at the center of Ring1, Ring2 is limited. Further, since the signal received at the center of the receiver coil is the strongest, and the signal becomes weaker as the distance from the center becomes larger to a point where it cannot be detected, the field of view (FOV) is thus limited.
In order to overcome the limitation of inversely connecting the loop coils from both sides, another method, utilizing a decoupling capacitor, is provided in Siemens Internal Document Part Number: 7100303, 7100394. FIG. 2 is a schematic diagram showing such a decoupling method. In FIG. 2, decoupling circuitry of an incorporated circuit of three independent loop receiver coils is shown, wherein there is a decoupling capacitor connected between any two loop coils, as represented by Ca12, Ca13, Ca23 respectively, and the connecting point of each decoupling capacitor and the incorporated circuit lies between the inductor and the capacitor in the detuning loop, i.e., between L1 and Ca1, L2 and Ca2, L3 and Ca3 as shown in FIG. 2. Since the magnetic signals emitted from human body are received simultaneously, the direction of the current in the first coil LOOP 1 and in the second coil LOOP 2 is identical, which may enhance the signal at the center, and the signal-to-noise ratio (SNR) in the center of the receiver coil may thereby be increased. Moreover, since the coils at the two sides are two separated coils, and a voltage is applied to each coil, the signal strength received is high, and the field of view is naturally increased.
However, there are still some problems in this method. As seen in FIG. 2, the decoupling capacitor Ca12 is grounded via the capacitor Ca2, while the capacitor Ca1 has one end connected to the decoupling capacitor Ca12 and the other end grounded. Thus, the serial loop formed by the decoupling capacitor Ca12 and the capacitor Ca2 can be considered as being connected in parallel with Ca1. In other words, the detuning circuit of the first receiver coil LOOP1 at this time is composed of the above parallel circuit and L1, rather than Ca1 and L1 in an ideal state. This will lead to a change in the original detuning frequency so that it is no longer the characteristic frequency of the system, and as a result, the receiver coil cannot be completely shut off when the transmitter coil is transmitting signals. Therefore, the receiver coil will generate some interference to the signals transmitted by the transmitter coil and offset some of them. The transmitting power of the transmitter coil must be increased to compensate the signals offset, i.e., by increasing the voltage of the MRI system. Ideally, it is intended that the receiver coil be shut off completely, i.e. the receiver coil does not operate, when the transmitter coil is transmitting signals. Moreover, since the decoupling capacitors Ca12, Ca23, Ca13 are connected to the two receiver coils respectively in this decoupling method, the capacitor in each receiver coil will exert an influence on the other receiver coil. This causes the capacitor in a receiver coil to be connected, and interact with other receiver coil loops through the decoupling capacitor. Therefore, if the detuning loop of a receiver coil loop operates, the decoupling of the other two coils will be affected.
Another known decoupling method is shown in FIG. 3—the theory underlying this known technique is to perform decoupling at the other end of the ground conductor. The connecting points of the decoupling capacitors Cb12, Cb13, Cb23 with the receiver coil array circuits are at the ungrounded sides of the tuning capacitors Cb1, Cb2, Cb3 in the tuning loops of the receiver coil circuits LOOP1, LOOP2, LOOP3. Since Cb12, Cb13, Cb23 can eliminate the inductive or capacitive coupling existing in the circuit, decoupling can be realized.
Disadvantages of this method are as follows. First, since the coupling may be inductive, capacitive or resistive, although this method may offset the inductive coupling and the capacitive coupling to some extent, it cannot compensate the resistive coupling. Second, since conversions between the electric field and the magnetic field exist in the radio frequency field: if the orientation of the electric field changes, the orientation of the magnetic field changes correspondingly, which means a transition between the susceptance and the reactance in the circuit. Therefore, if the direction of the decoupling in the circuit is incorrect, the capacitance will be changed to an inductance with opposite character; or when an inverter is added, since the inductance will introduce certain resistance, additional loss will be brought to the circuit.